This invention relates generally to cryogenic air separation and, more particularly, to cryogenic air separation for producing argon.
In the production of argon by cryogenic air separation the actual recovery of argon in a plant is often reduced well below design levels due to operational concerns with the nitrogen tolerance of the crude argon condenser. Specifically, as the relative concentration of nitrogen increases at the top of the crude argon column (condensing side of the overhead condenser), the temperature required to completely liquefy the gas phase decreases. The lower limit of this condensing temperature is set by the minimum temperature of the refrigeration source as well as the heat transfer and flow characteristics of the condenser. When the amount of nitrogen present on the condensing side is great enough, a portion remains uncondensed. Unless it is withdrawn, the presence of this uncondensed nitrogen gas begins to drive down the required condensing temperature. A nitrogen gas buildup can rapidly reduce the amount of gas that can be liquefied. Since it is the condensing action that draws the feed flow into the bottom of the crude argon column, a reduction in the quantity of gas condensed causes an equal reduction in the column feed flow. With a significant reduction of column feed flow, the liquid on the distillation stages will not be properly supported by the rising gas so excessive amounts of liquid will then fall to the column sump. This loss of gas feed and resultant liquid dumping causes the crude argon column to stop working. This usually leads to a severe upset in the lower pressure column with which the crude argon column is integrated. In order to avoid this rapidly occurring nitrogen induced upset, especially during plant capacity changes, prepurifier bed switches or other operating mode changes, the crude argon column feed flow is often controlled to maintain its nitrogen concentration at a low value. Unfortunately, the consequence of maintaining the nitrogen at a low value means that the argon concentration as well as the total flow rate of the crude argon column feed stream are also maintained at a low value. Since only the argon actually drawn into the crude argon column has a chance of being recovered, this leads to a reduction in the argon production.
Accordingly, it is an object of this invention to provide a cryogenic air separation method wherein argon production may be increased.
The above and other objects, which will become apparent to those skilled in the art upon a reading of this disclosure, are attained by the present invention which is:
A method for producing argon by cryogenic rectification comprising:
(A) passing feed air into a higher pressure column of a cryogenic air separation plant which also comprises a lower pressure column and an argon column having a top condenser, and separating the feed air by cryogenic rectification within the higher pressure column to produce oxygen-enriched liquid and nitrogen-enriched vapor;
(B) passing argon-containing fluid from the lower pressure column as feed into the argon column and producing crude argon vapor by cryogenic rectification within the argon column;
(C) withdrawing oxygen-enriched liquid from the higher pressure column and mixing liquid nitrogen with oxygen-enriched liquid withdrawn from the higher pressure column to produce a liquid refrigeration mixture;
(D) condensing at least some of the crude argon vapor by indirect heat exchange with the liquid refrigeration mixture in the argon column top condenser to produce crude argon liquid and vaporized refrigeration mixture;
(E) passing vaporized refrigeration mixture from the argon column top condenser into the lower pressure column; and
(F) recovering some of at least one of the crude argon vapor and crude argon liquid as product argon.
As used herein the term xe2x80x9cfeed airxe2x80x9d means a mixture comprising primarily oxygen, nitrogen and argon, such as ambient air.
As used herein the term xe2x80x9cliquid nitrogenxe2x80x9d means a liquid having a nitrogen concentration of at least 60 mole percent.
As used herein the term xe2x80x9ccolumnxe2x80x9d means a distillation or fractionation column or zone, i.e. a contacting column or zone, wherein liquid and vapor phases are countercurrently contacted to effect separation of a fluid mixture, as for example, by contacting of the vapor and liquid phases on a series of vertically spaced trays or plates mounted within the column and/or on packing elements such as structured or random packing. For a further discussion of distillation columns, see the Chemical Engineer""s Handbook, fifth edition, edited by R. H. Perry and C. H. Chilton, McGraw-Hill Book Company, New York, Section 13, The Continuous Distillation Process. The term, double column is used to mean a higher pressure column having its upper end in heat exchange relation with the lower end of a lower pressure column. A further discussion of double columns appears in Ruheman xe2x80x9cThe Separation of Gasesxe2x80x9d, Oxford University Press, 1949, Chapter VII, Commercial Air Separation.
Vapor and liquid contacting separation processes depend on the difference in vapor pressures for the components. The high vapor pressure (or more volatile or low boiling) component will tend to concentrate in the vapor phase whereas the low vapor pressure (or less volatile or high boiling) component will tend to concentrate in the liquid phase. Partial condensation is the separation process whereby cooling of a vapor mixture can be used to concentrate the volatile component(s) in the vapor phase and thereby the less volatile component(s) in the liquid phase. Rectification, or continuous distillation, is the separation process that combines successive partial vaporizations and condensations as obtained by a countercurrent treatment of the vapor and liquid phases. The countercurrent contacting of the vapor and liquid phases is generally adiabatic and can include integral (stagewise) or differential (continuous) contact between the phases. Separation process arrangements that utilize the principles of rectification to separate mixtures are often interchangeably termed rectification columns, distillation columns, or fractionation columns. Cryogenic rectification is a rectification process carried out at least in part at temperatures at or below 150 degrees Kelvin (K).
As used herein the term xe2x80x9cindirect heat exchangexe2x80x9d means the bringing of two fluids into heat exchange relation without any physical contact or intermixing of the fluids with each other.
As used herein the term xe2x80x9ctop condenserxe2x80x9d means a heat exchange device that generates column downflow liquid from column vapor. The top condenser may be physically within or may be outside the column.
As used herein the terms xe2x80x9cturboexpansionxe2x80x9d and xe2x80x9cturboexpanderxe2x80x9d mean respectively method and apparatus for the flow of high pressure gas through a turbine to reduce the pressure and the temperature of the gas thereby generating refrigeration.
As used herein the terms xe2x80x9cupper portionxe2x80x9d and xe2x80x9clower portionxe2x80x9d means those sections of a column respectively above and below the mid point of the column.
As used herein the term xe2x80x9csubcoolingxe2x80x9d means cooling a liquid to be at a temperature lower than that liquid""s saturation temperature for the existing pressure.